-0.45%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 215/60R17, fitting 255/45R18 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 39.8 km/h | 25 mph | 25 mph |
| 60 km/h | 59.7 km/h | 37 mph | 37 mph |
| 80 km/h | 79.6 km/h | 50 mph | 49 mph |
| 100 km/h | 99.6 km/h | 62 mph | 62 mph |
| 120 km/h | 119.5 km/h | 75 mph | 74 mph |
Full comparison
215/60R17
Overall diameter689.8 mm · 27.16 in
Section width215 mm · 8.46 in
Sidewall height129.0 mm · 5.08 in
Circumference2167.1 mm · 85.3 in
Revolutions per mile743
Revolutions per km461
255/45R18
Overall diameter686.7 mm · 27.04 in
Section width255 mm · 10.04 in
Sidewall height114.8 mm · 4.52 in
Circumference2157.3 mm · 84.9 in
Revolutions per mile746
Revolutions per km464
-0.45%Safe swap — speedometer error is minor
Original
Overall diameter689.8 mm · 27.16 in
Sidewall height129 mm
Circumference2,167.1 mm
Revolutions / mile742.6
New
Overall diameter686.7 mm · 27.04 in
Sidewall height114.8 mm
Circumference2,157.3 mm
Revolutions / mile746
Speedometer & odometer error
The new tire changes overall diameter by -0.45%. With a speedometer calibrated to the original tire, that same percentage is how far the speedometer and odometer read off — a taller tire means your true speed is higher than indicated and the odometer under-counts distance.
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 39.8 km/h | 25 mph | 25 mph |
| 60 km/h | 59.7 km/h | 37 mph | 37 mph |
| 80 km/h | 79.6 km/h | 50 mph | 49 mph |
| 100 km/h | 99.6 km/h | 62 mph | 62 mph |
| 120 km/h | 119.5 km/h | 75 mph | 74 mph |
See individual specs: 215/60R17 · 255/45R18. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.